It has been previously proposed to investigate subterranean formations in situ by observing the propagation of radio signals. Generally, a variety of different processes and apparatus have been proposed; however, although considerable worthwhile data has been provided, a need continues to exist for an effective system having improved accuracy and relative economy.
In general, the present system employs radio signals of low frequency that are broadcast from one or more transmitter locations. For example, commensurate signals of different frequencies are transmitted from a single location to a receiver that is located above a formation of interest. At each point along the path of the wave, part of the energy enters the earth and propagates in a direction almost vertically downward. Such radio energy is reflected from the formation, as upon encountering an impedance discontinuity. Accordingly, energy returns to the surface, to be sensed by a receiver.
Signals of longer wavelength (lower frequency) penetrate deeper into the formation than the shorter wavelength (higher frequency) signals. Accordingly, different paths are traveled by signals of different frequency and, accordingly, manifest phase variations at the radio receiver. The received signals are reduced to a phase-comparable form and phase detected to provide indications of the subterranean formation. In utilizing three or more different-frequency signals, considerably-improved depth-related data is provided. By employing signals from a plurality of locations, improved illumination of the formation results in more-significant data.